I personally would never pay for A/V, there are plenty of decent FREE products out there, AVG, Avira etc to name a couple.

I have never used Kaspersky, despite being able to for free so don't know the product, but if it slows your machine down then it's most likely has high resource utilization either in terms of CPU/Memory usage.

As to scanning it depends on two things mainly, The volume of data you have, and the spec of your system, that coupled on with the type of scan you are performing, i.e. quick, full etc.

You could probably configure Kaspersky to be faster by turning off the add-ins etc to stop it interacting with your web browser, it's going to be whatever it is performing in the background etc that is impacting on the performance.

Martin